Downloaded the firmware update t65x.zip, Access Code "t65x". Unzipped & ran the loader application Lexmark_T650_T652_Firmware_Update_Utility_P510_E217.exe but it could not find the printer over the network. Tried on Windows 7 and Windows XP (VMs) w/o success. Instead the firmware update was performed via the printer's web interface as follows (copy&pasted from the t65x.zip readme file);

To update the printer's firmware, perform the follow steps:
1. Confirm the printer displays Ready.
2. On a computer attached to a network, open a Web browser.
3. Enter the printers IP address in the Web browsers address bar.
4. Press Enter. The printer's homepage opens.
5. Click Settings. The settings page opens.
6. Click the Update Firmware link. The update firmware page opens.
7. Click the Browse button. A file browser will open.
8. Browse to T65x_LRJPa_P510_E217.fls.
9. Click Open.
10. Click Submit. Various status and progress messages will appear on the operator panel. The printer will
reboot. The webpage will post File Transfer Complete. The HTTP session will terminate.
Warning: Do not turn the printer off until the update is complete. When the firmware update is completed,
the printer will automatically reboot and return to a Ready state.

Note: The flashing/updating process is very slow, taking > 1hour. The updated firmware is now;